The tropics as a prime suspect behind the warm-cold split over North America during recent winters

This is a guest post by Dennis L. Hartmann, who is a professor in the Department of Atmospheric Sciences at the University of Washington in Seattle. He literally wrote a textbook that was the foundation for our climate training (webpage and CV), so we’re pretty enthused he’s guest blogging with us—to put it mildly. Still, the Climate.gov managing editor asks us to remind our readers that all blog posts convey the opinions and ideas of individual bloggers, not NOAA.This post offers one expert’s view on a question that many scientists are exploring and debating.

So far this winter it has been warm and dry in the West and cold and snowy in the East. As I write from Seattle, it is sunny and in the mid-fifties, and the Olympic Mountains have very little snow on them for late winter. Meanwhile it is 25°F in Chicago, and Boston is bracing for another snowstorm that might bring their total closer to the snowiest season on record.

This winter bears some similarity to the record-breaking “Polar Vortex” winter of 2013-14, which was the second coldest on record for Chicago and was associated with an unusual seasonal pressure pattern that contributed to drought in California (Seager et al. 2014).

Patterns of SST variability

Figure 1 shows the circulation anomalies (departures from average pressure as measured by the 500hPa geopotential height field) for the winters of 2013-14 and 2014-15, so far. Over the Pacific and North America for both winters, this pattern favors warm and dry in the West and cold in the East, with some differences in details.

People have speculated that this weather ‘weirding’ might have to do with global warming, either through the Arctic Sea Ice decline (Francis and Vavrus 2012), or through warming of the tropical oceans (Palmer 2014; Wang et al. 2014). I will argue here that these anomalies can be seen in past patterns of natural variability, and are likely caused by sea surface temperature changes in the tropics. I will not address the question of whether the probability or intensity of this pattern might be influenced by global warming.

The two most common sea surface temperature correlation patterns in Pacific Ocean north of 30°S over time, based on EOF analysis. The first is the classic ENSO signal, while the second is the North Pacific Mode. The contour interval is 0.1, and the zero contour is white. Red and blue show correlations between anomalies of opposite sign. When red areas have above-average SSTs, blue areas have below-average SSTs, and vice versa. See Hartmann (2015) for regression plots in units of ˚C.

While ENSO accounts for much of the tropical ocean-atmosphere variability that dominates climate from year-to-year, classic ENSO indices do not account for everything. In addition to the classic ENSO signal of sea surface temperature (SST) variability, decadal signals of SST variability exist that exhibit strong coupling between the tropics and higher latitudes (Hartmann 2015). Figure 2 shows the two most important patterns of SST variability for the Pacific Ocean region north of 30S. The first of these is the classic ENSO pattern, while the second can be called the North Pacific Mode (2)(3).

The North Pacific Mode pattern consists of above-average SSTs in the western Tropical Pacific that extend north along the California coast and across the far northern Pacific Ocean. It is thus a pattern of natural variability in the coupled ocean-atmosphere-land system that connects the tropical and middle latitudes. While ENSO has been in a neutral state for the past few winters, the NPM has been in an extreme positive state since the summer of 2013 (Figure 3).

Monthly time series of ENSO (top, blue line) and North Pacific Mode (NPM) (below, orange line) in the Pacific Ocean north of 30°S from January 1979 to January 2015. Units are in standard deviations from the mean. The NPM has been highly positive since the summer of 2013. Graph by Fiona Martin, based on data provided by Dennis Hartmann.

The downstream influence of the North Pacific Mode

The North Pacific Mode SST pattern (Figure 2 bottom) is associated in the observational record with a 500hPa height anomaly pattern that looks very similar to the seasonal anomaly pattern from the winters of 2013-14 and 2014-15, with a ridge along the Rockies and a deep trough over the middle of North America (compare Figure 4 (left) with Figure 1). This pattern brings warmth and drought to the West and cold to the East, as we have observed the past two winters.

Of course, it’s possible for the patterns of SST and 500hPa height to be associated—to occur at the same time—without one being the cause of the other. But by using the observed SST under atmospheric models and performing many test simulations of the period from 1979 to the present, it can be shown that the SST anomalies associated with the North Pacific Mode do drive the “downstream” pressure anomalies over North America (Hartmann 2015). Figure 4 (map on right) shows that the simulated 500hPa anomalies produce a pattern that is very much like the observed one (compare Figure 4 left and right) (4).

We therefore know that the SST anomalies in the Pacific were a prime cause of the “Polar Vortex” winter of 2013-14. Since the SST anomaly pattern persisted into this winter (Figure 3), it is reasonable to suppose that the SST anomalies are also contributing to the warm, dry West and cold East anomalies experienced so far in 2015.

Regression analysis showing pressure anomalies associated with variability in the North Pacific Mode since 1979 (from Figure 2). Observed 500hPa height anomalies during the positive phase of the NPM, based on NCEP reanalysis data (left). Simulated pressure anomaly patterns from models (ESRLv2 50-member model ensemble) forced by sea surface temperature patterns derived from the same NPM time series (right). Maps by NOAA Climate.gov, based on data provided by Dennis Hartmann.

Based on many observational and modeling studies, we know that seasonal anomalies in SST and weather in the midlatitudes can be driven by tropical SST anomalies. The tropical ocean area is large, and the atmospheric heating is sensitive to tropical SST anomalies (Lau and Nath, 1994). My colleagues Paulo Ceppi at the University of Washington and Peter Watson at Oxford University have run some initial model experiments indicating that the observed SST anomalies in the tropical Pacific Ocean cause the extratropical height anomalies of the winter of 2013-14 in models. We therefore believe that the anomalous weather in North America during the winters of 2013-14 and 2014-15 is related to the warm SST anomalies in the western tropical Pacific (as part of the North Pacific Mode pattern shown in Figure 2), which have persisted since the middle of 2013.

Although the North Pacific Mode is known to be a product of natural variability associated in some way with ENSO, this mode of variability has become more prominent since 1979. Whether the enhanced importance of this mode is related to natural variability, global warming, or just changes in observing systems, is, I think, unknown at this point.

The CMIP3 and CMIP5 global warming simulations (5) move toward a more El Niño-like state in the eastern equatorial Pacific where, on average, the models warm the SST more than in the tropics as a whole. The models are not in agreement on this, however, and there is low confidence in changes in the intensity and spatial pattern of El Niño in a warmer climate (Christensen et al. 2013).

Meanwhile, a major El Niño has not occurred since 1998, but some strong La Niñas have (Figure 3), so that nature has moved more toward a La Niña state over the past 15 years. So while we are fairly sure that tropical SSTs are the apparent cause of the unusual nature of our past couple of winters, we do not know for sure whether this is just part of the natural variability of climate, or whether climate change is favoring the positive phase of the North Pacific Mode of SST variability.

Footnotes

(1) The most common patterns are determined by EOF analysis, which is short for Empirical Orthogonal Function. EOFs are statistical ways to identity the pattern and time series that is associated with the largest amount of variability. The pattern that is associated with the largest amount of variability is often referred to as EOF/Principal Component-1 or as the leading pattern of variability. This analysis used standardized data.

(2) I have used correlation patterns here rather than regressions showing temperatures in degrees to better reveal the connections to the tropics, where small changes in temperature can be more important because the temperature is already high and the highest temperatures are “felt” by the global atmosphere. In other words, we want to look at the strength of the relationship between SST in different locations more than the actual amount of the temperature variation involved.

(3) The North Pacific Mode (NPM) described here is distinct from both the Pacific Decadal Oscillation (PDO) and the El Niño-Southern Oscillation (ENSO). The NPM is often in its positive phase as shown in Fig. 1b just prior to an ENSO warm event, but not always. It is related to the ‘seasonal fingerprinting’ mechanism (Vimont et al. 2003) whereby tropical-extratropical interactions can set the stage for El Niño.

(4) The degree of similarity between the simulated and the observed patterns is determined objectively by applying a regression of the simulated 500hPa anomalies onto the time series of the observed NPM SST model. The modeled response is a little weaker and is missing some components in the far north, but the structure of the wave train that affects the United States is well simulated by simply specifying the observed SST.

(5) CMIP stands for the Couple Model Intercomparison Project and studies the differences in coupled atmosphere-ocean general circulation models. CMIP3 refers to the third phase of the project and was used as new data in the IPCC Fourth Assessment. CMIP5 is the fifth and most recent set of simulations that was used for the IPCC Fifth Assessment.

References:

Christensen, J. H., and Coauthors, Eds., 2013: Climate Phenomena and their Relevance for Future Regional Climate Change. In: Climate Change 2013: The Physical Science Basis, Contribution of Working Group I to the Fifth assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press.

A blog about monitoring and forecasting El Niño, La Niña, and its impacts.

Disclaimer:

The ENSO blog is written, edited, and moderated by Michelle L’Heureux (NOAA Climate Prediction Center), Emily Becker and Tom Di Liberto (contractors to CPC), Nat Johnson (NOAA Geophysical Fluid Dynamics Laboratory), and Rebecca Lindsey (contractor to NOAA Climate Program Office), with periodic guest contributors. Ideas and explanations found in these posts should be attributed to the ENSO blog team, and not to NOAA (the agency) itself.

Every year is a little different, and Nov-March 1995-6 (winter of 1996) also has other differences like a non-zero third mode, but if I look for a trough along 35N in the W. Pacific, a ridge over the Bering Sea, and a trough over the eastern half of North America, I see those anomalies then. A weak ridge also appears centered over California. The winter of 1997 (Nov. 1996- March 1997) has the Pacific and California features, but the ridge does not extend down the West Coast and the trough over North America is not well developed. Other years that appear to have this pattern are 1991, 1994 and 2003, but none as dramatically as 2014. It is also possible to find years that have this pattern with the opposite polarity, 1995, 1999, 2000 and 2006, to varying degrees, and just by eye, nothing quantitative. You have to bear in mind that the correlations between the NPM and the monthly mean height field in winter are only about 0.3, so the fraction of variance explained is small.

Thank you for your response and this interesting article. It might be interesting to model this effect in conjunction with a weakening jet stream as proposed by Jennifer Francis to see if the correlation is expected to become stronger (or maybe weaker) in a warming environment.

Could this Warm Temperature Blob, along the west coast of North America, be caused by the huge amount of leaked radiation from Fukishima Nuclear Disaster? I know my comment is most likely way out of context with this original comment, but I think this is most likely the cause of the Warm Temperature Blob!

The causes of the warm SST anomaly are discussed in a new paper by Bond, Cronin, Freeland and Mantua.http://onlinelibrary.wiley.com/doi/10.1002/2015GL063306/abstract
They show that anomalously high atmospheric pressure in middle latitudes leads to less cooling of the ocean water during the winter season. The atmospheric high pressure anomaly is, I have argued, organized by the SST anomalies in the Tropics.

But let's think about the energy involved in warming the North Pacific. The warm anomaly extends over an area of about 1000 by 3000 kilometers and is about 100 meters deep. To warm that volume of water by one degree requires about two million Tera Joules of energy. The two nuclear bombs dropped on Japan during WWII totaled about 147 Tera Joules, so they would provide a negligible fraction of the energy necessary to warm the ocean. During the single month of July the sun makes available about five million Tera Joules to the same patch of ocean. So heating from Fukishima can do very little to affect the temperature over such a large mass of ocean, in comparison to the natural processes that move solar energy through the climate system.
By changing the greenhouse gas composition of the atmosphere, though, humans can affect the flow of energy through the climate system and influence the global surface temperature that way, just to be clear. (A Tera Joule is 10^15 Joules. A Watt is a Joule per second.)

Well, clouds directly affect how much of the available solar radiation makes it to the ocean. They can reflect about half of it, depending on how thick the clouds are and how much of the ocean they cover. So what clouds do is important. In the eastern equatorial Pacific during El Niño, when the SST warms up there, low clouds are replaced by high clouds, so it is not as if you are adding clouds where none were before.

Dennis
The image showing the NPM I confused with a positive PDO. Also the NPM image shows a cool SST anomaly in the equatorial region of the Eastern Pacific which I assumed was more indicative of La Niña. Could you explain the difference between PDO and NPM. Also could you clarify how the NPM image does not show a week La Niña signature.
Thank You

Brody,
The PDO is defined as the first EOF of SST anomalies north of 20N in the Pacific Ocean. If I do that analysis, the second mode is the NPM. This is discussed in my GRL article. Check out the supplementary materials in GRL, if you can find them. The NPM has a vaguely similar shape to the PDO, but is actually orthogonal to it. The shape of the NPM is closer to that of the Atlantic Multi-Decadal Oscillation, and is associated with the north-south dipole structure in the height field. What I show in the paper and this blog are the EOF's for SST anomalies north of 30S in the Pacific. In that case, for the recent data since 1979, the NPM is the second EOF and ENSO is the dominant first EOF. You do see some cold SST anomalies in the eastern equatorial Pacific, but I am not sure if that is critical to the primary nature of the mode, or just the EOF analysis trying to span the data as best it can. As a side note, one can now question whether doing analysis for the region north of 20N makes sense, since we know that the SST anomalies there are strongly driven from the tropics. A school of thought would say that the PDO is just the low frequency signature of ENSO in the North Pacific. If so, why not include the tropical Pacific in your thought process from the outset?

If I look at my own analysis of the SST, then 76-77 was a strong La Niña year, the warm blob was a bit farther south than in 2014. I was in Colorado that winter and there was not much snow in the Front Range. Fig. 9a of Miyakoda and Rosati (1984) shows a warm blob at about 35-40N and 150W, about where you expect warm SST associated with La Niña, and a cold anomaly exists to the north along the coast of Alaksa. The warm blob in 2014 and the NPM warm blob are a bit farther north and the warm SST anomaly extends all the way to the coast of Alaska. These subtle variations in positioning are what the two decadal modes pick up as leftovers from the ENSO mode.

Firstly, thanks for this research and it offers a valid theory for the current state of affairs. I agree with in principle with your findings, but want to know (and so do many colleagues I've shared this with) as to the continued feedback from the atmosphere to the ocean, i.e. that once the pressure anomalies occur, there is another set of feedbacks due to the winds at sea level due to initial feedback from the ocean to the atmosphere. Typically, stronger surface highs over the eastern N. Pacific will cause upwelling and cooler SST's owing to northerly winds along the West Coast, esp. of California. However, I do believe it's all related to the exact positioning of the surface high pressure, known locally as the RRR, or Ridiculous Resilient Ridge over the western United States.

Also, how to explain the massively above average rainfall of DEC 2014 (int he SF Bay area at least - 18.47" in Sausalito in Marin Co.) under this regime? Is it a case of a massive breakdown and breakthrough of the jetstream associated atmospheric river events? The last 2 winters have seen 4 atmospheric River events, which appears statistically above normal, and seemingly associated with the highly positive NPM. There was another AT even in FEB 2015. late 2013 and Feb 2014 both had AR events under this highly positive NPM regime.

First: The evidence so far suggests that the extratropical atmospheric anomalies are coordinated by tropical SST anomalies. The extratropical wind anomalies drive extratropical SST anomalies. Most evidence from modeling says that those extratropical SST anomalies have only a weak feedback on the atmospheric flow, but I think this is a question that could use some further study.

Second: This is just speculation, but when the NPM is in full flower, warmer than normal SST's extent to the west and south of California, which is were the atmospheric river events come from. So if the pattern does break down temporarily into a low that brings rain to Califormia, then it might be more than in some other year when the SST upstream is colder.

Thank you for the article. My question is: can you comment on how the NPM has changed specifically over the last two years? Did its appearance explicitly precede the North American dipole characteristic? Does it appear to be waning, amplifying, or moving in any way? Thanks.

I am not sure I have studied this question exactly, but let me tell you what I said in my GRL article. The NPM mode of SST gets large and positive during the summer of 2013, about May. This is before the winter of 2013-14. The NPM anomaly has stayed large at least through January of 2015. I expect it to give way eventually. If it stays for another winter, that will be a real news story, I think.

You write: "I will not address the question of whether the probability or intensity of this pattern might be influenced by global warming."
Why? What does that mean?
That perhaps it's possible to warm up our global climate system and not see "natural" patterns mutate and intensify into harmful "climate change" ?
Sorry, but I find it appalling the way so many scientists have been kowtowed but right wing bullies into shooting themselves in the foot with every attempt to inform the public. This isn't a quaint scholastic debate! We are talking about the observed rapid degradation of the world our children will inherit and pretending AGW 'might' not be radically altering ancient patterns is unacceptable.

I think by including that comment I am signaling that it is possible that the greater emergence of this pattern could be associated with human-induced climate change. From scientific perspective, however, I think it will take a lot of careful work to prove or disprove that hypothesis. The SST pattern and the extratropical mode of variability that it triggers in the cases of the winters of 2014 and 2015 are both natural modes of variability that existed before the rapid warming between 1970 or so and the present. It is probably true that neither winter is unprecedented. The average of the coupled climate model projections of a greenhouse gas warmed future go toward more warming in the tropical East Pacific than West Pacific, although there is no consensus among the models on this. So no firm prediction of how the tropical SST will change in a warmed Earth can be made on the basis of the ensemble of climate models. Some theories predict more La Niña-like and some predict more El Niño-like responses to warming. What we observe in 2014 through 2015 are warm anomalies in the far western Pacific and slight cooling in the East Pacific. Not exactly a classic La Niña pattern, but in the same direction, with warming to the west. Is this a trend, or just an expression of the rich natural variability of the climate system? I don't know.

Human-induced climate change is real, ongoing, and a serious threat to humans and our natural and built environments. Everything is changing. This has been clearly communicated by the scientific community in numerous reports at the international, national and local levels. Of course it worries me that reaction to these facts by decision makers has been slow and inadequate, but I believe it would be a mistake for me to respond to this by cutting corners on the scientific method. I feel the best role for scientists is to try to communicate what is known and what is possible and try to calculate and communicate the probabilities. If we are to be helpful, we need to be trusted. Eventually the truth wins, I hope not too late.

I am a skeptic and Dennis Hartman's discussion made me trust him as a scientist with integrity. I believe CO2 has some impact but I am not sure how sensitive the climate is to CO2 nor how catastrophic that sensitivty may be.. To evaluate climate sensitivity we must present an honest accounting of natural climate variability. I lost trust in all climate scientists who cut corners and simply blame everything on CO2 warming when natural variability provided and equally plausible explanation. The skeptics I know objectively evaluate climate science hypotheses and like myself have more trust and give more weight to those scientists who carefully evaluate contributions from natural climate dynamics. Dennis Hartman has gained my trust. If there were more who did not cut corners there would be a more conducive atmosphere for sincere climate discussions.

Thank you for this interesting research. I live in Europe and I noticed that the winter 2013/14 was very strange, because of the persistence of wet, mild conditions in many parts of the continent, probably a consequence of the 2 wave pattern around the northern hemisphere in association with the extreme +ve NPM.
I have a suggestion in response to citizenschallenge and a question for Dennis.
Suggestion: as Kevin Trenberth suggests, I think that instead of questioning if the probability or intensity of a pattern might or not be influenced by global warming, if is it due to global warming or to natural variability, could be better to assume that it could be both. I think that changes in atmospheric circulation is a product of internal, free variability (as spontaneously occurring changes in the amplitude and polarity of preferred atmospheric circulation patterns) but this product can carry an externally forced dynamically response (think for instance the changes in the wintertime circulation over high northern latitudes induced by large volcanic eruptions or the widening of the tropical Hadley cells in response to human-induced global warming or in response to changes in solar irradiance). The challenge is to find if/how much this is completely internal or/and forced.
Question: I read (for instance Seager et al. 2007) that the Medieval Climate Anomaly - which was connoted by a series of pluri-decadal severe droughts that afflicted western North America - was part of a pattern characterized. among other things, by a La Nina-like tropical Pacific and by the warm phase of the Atlantic Multidecadal Oscillation. If I rememberer well, reconstructions tell us that even the IPO was in her -ve phase. Do you think that this particular period might well have been influenced by the NPM, too?

Regarding your first point, A theorem from statistical mechanics states that we should expect the first signs of forced change to appear in the free modes of variability of a system. So change might look like natural variability that gets a little large or hangs around longer than normal.

I don't know much about the Medieval Climate Anomaly, so don't feel comfortable trying to answer your question. The main El Niño mode is clearly the big dog, but the NPM and other structures also play a big role at times.

What is the relationship between the North Pacific Mode and the~18 year North Pacific Gyre Oscillations? Because the NPGO captures changes in salinity, might it not also represent stored heat affecting te NPM as warmer saltier water lurks below the surface and its ventilation would be sensitive to the intensity of wind turbulence?

Thank you for explaining NPM and its relation with ENSO and decadal modes. If I'm understanding correctly from the earlier comment, Winter 1976-1977, the primary culprit for the cold extremes in the late 1970s was La Niña, whereas the primary culprit the past two winters was NPM? Does anyone have an idea why NPM does not always precede El Niño (as described in footnote 3)?

Is there any known correlation between NPM and Judah Cohen's hypothesis regarding downstream effects of increasing Siberian snowfall in autumn? Cohen's hypothesis goes something like this, and sounds like a special case of Jennifer Francis's arctic amplification scenarios: Warming global temperatures > extreme Eurasian snowfall in autumn > cold dome near Altai, Himalayas, and Tien Shan > amplification of Rossby waves in Pacific, both stratospherically and longitudinally > more meandering jet stream over North America that winter > lobes of polar vortex frequently breaking off and heading into eastern USA.

No, I have not. It takes a few hours work. It might be interesting to dot. One thing that did happen is that the big precursor mode (NPM or Blob Mode) did result in a big El Niño, as past history suggests it should have. We should probably then get into a strong La Niña state, which also seems to be happening. I am curious whether the negative PDO we've had will also reverse, or whether the negative PDO will return.

Great stuff. Thanks for the discussion -- tremendously helpful in trying to sort out the different players in these complex scenarios.

In regard to your last comment -- La Niña seems to be on the way, said NOAA, but I haven't seen any discussion of its amplitude, nor heard much discussion re: the PDO. As we head towards fall, could you comment on any recent developments on either front?